1. Field of Invention
This invention relates generally to an apparatus and method for determining the minimum miscibility pressure (MMP) for a gas in a liquid, and in particular, to an apparatus and method for determining the minimum miscibility pressure for a gas in a liquid based on pressure changes in a rising bubble apparatus.
2. Description of Related Art
An enhanced oil recovery process is a process for increasing the production of oil from a subterranean formation by injecting a liquid or a gas into the formation in a quantity sufficient to flood the formation. When designing a gas flood, it is important to determine the minimum pressure at which the flood gas is miscible in the reservoir oil. Conducting a gas flood at or slightly above the MMP increases oil production because the capillary forces are eliminated and the oil is displaced as a single phase. Also, when the gas is dissolved in the oil, the oil volume increases and the viscosity decreases. If the formation-pressure is less than the MMP, the pressure must be increased before miscible oil displacement can occur. However, increasing the pressure of a gas injected into a subterranean formation pressure substantially above the MMP increases operating costs, increases the risks of undesirable formation fracturing, and creates additional hazards for personnel, without significantly improving oil production. Thus, it is desirable to perform laboratory experiments to determine the MMP of the desired flooding gas .in the oil at the temperature of the formation.
One method for determining the MMP of a system comprising a gas and a liquid is to observe the behavior of gas bubbles rising through the liquid. U.S. Pat. No. 4,627,273 to Richard L. Christiansen and Hiemi Kim describes an apparatus and method for determining the minimum miscibility pressure (MMP), the pressure at which a gas and a liquid are miscible. A single gas bubble is injected into a non-horizontal, transparent glass tube containing the liquid at a fixed temperature and a predetermined pressure. The behavior of the bubble is observed as it rises through the liquid. Additional observations are made at a series of pressures from low to high, to determine the lowest pressure at which a bubble exhibits characteristic multiple contact miscibility behavior.
Initially, the gas bubble is nearly spherical. If the pressure is below the MMP, the top of the bubble remains spherical, and the bottom interface of the bubble becomes flat. The gas bubble retains its nearly spherical shape as it rises, and the size of the bubble decreases as material is transferred from the bubble to the surrounding liquid. As the pressure is increased to approach the MMP the flatness of the bottom interface of the bubble vanishes, and the tail-like features develop along the sides and/or the bottom of the bubble. When the pressure is slightly below the MMP, the contents of the bubble quickly disperse in the liquid, and the bottom interface of the bubble severely deteriorates. If the pressure is at or slightly above the MMP, the gas is multiple contact miscible in the liquid, and the full miscibility is achieved in stages. The bubble can not retain its integrity and breaks up into two or more smaller bubbles, which eventually dissipate, upon rise in the liquid.
However, the method of Christiansen and Kim is sometimes difficult to apply because the bubbles may rise too rapidly for easy observation. Further, observations are subjective, and the quality of the results depends upon the skill of the operator. If the apparatus is to be used with opaque liquids, it can be difficult to construct. For example, the tube must be flattened to facilitate visibility of the bubble, and the bubble injection mechanism must be centered to prevent growing bubbles from contacting the walls of the tube prior to launch. The need to use a transparent glass tube also limits the maximum temperature and pressure at which measurements can be made. In addition, it is desirable to make more accurate MMP determinations.
Accordingly, an object of the present invention is to provide an apparatus and method which allow more objective MMP determinations that are less dependent than current methods upon the operator's skill.
A further object of the present invention is to provide an apparatus which is simpler to construct and which allows measurements at high temperatures and pressures.
Yet another object of the present invention is to provide an apparatus and method with which more accurate results can be obtained.